Irradiation device for testing objects coated with light-sensitive paint

ABSTRACT

The invention concerns an irradiation device for testing objects coated with light-sensitive paint, comprising a EUV radiation source, an optical system for filtering the radiation of the EUV radiation source a chamber for receiving the object, as well as systems for intersecting the trajectory of the rays on the object. The invention also concerns a method for operating such a device. The invention aims at obtaining as quickly as possible an illumination at least partly simultaneous of several irradiation fields, with different doses, by using an inexpensive laboratory radiation source without resorting to complex optical systems. Therefor, the invention provides a device comprising a simplified and compact optical system, with closable diaphragm apertures located in front of the object to be irradiated and at least one control sensor placed on the trajectory of the rays and enabling the radiation dose to be measured.

The invention relates to a device for the test irradiation of objectscoated with photosensitive resists having an EUV radiation source, anoptical system for filtering the radiation from the EUV radiationsource, a chamber for receiving the object and also means forinterrupting the beam path onto the object. The invention additionallyrelates to a method for operating such a device.

The term lithography denotes, in semiconductor technology, a method fortransferring circuit patterns of microelectronic components andintegrated circuits onto a silicon semiconductor slice, the wafer. Forthis purpose, firstly a mask is produced which contains the pattern inthe form of differences in transparency for the beams which are used totransfer said pattern onto the wafer. The wafer surface is coated with aradiation-sensitive photoresist and exposed through the mask.Semiconductor structures are transferred onto the photoresist, by meansof a so-called lithography scanner. During the subsequent development,depending on whether a positive or negative resist is involved, theexposed or unexposed photoresist is dissolved away and the wafer surfaceis uncovered at these locations.

On account of the decreasing feature size of semiconductors, thefabrication of modern semiconductor elements, such as, for example,memory chips and CPUs, requires a resolution which makes it necessary touse extremely short-wave radiation of approximately 13 nm with a quantumenergy of approximately 92 eV (EUV radiation). The irradiationwavelengths of 248 nm (UV radiation), 193 nm (DUV radiation) or 157 nm(VUV radiation) used heretofore no longer suffice to produce theshrinking structures. As the feature size and wavelength decrease,however, there is an increase in the requirements made of the resistsused, the so-called resist material, as far as both the sensitivity andthe line roughness are concerned.

The changed requirements made of resists require the test systemsthereof to be adapted, said test systems being used to determine theresist properties with varying irradiation before series production ofthe wafers.

EUV radiation is absorbed by matter to an extremely high degree. It isnecessary, therefore, for the EUV radiation to be guided underultra-high vacuum conditions. The source of the EUV radiation is athermally emitting plasma. In contrast to the lasers used heretofore,plasma emits in a very broad band, so that DUV, VUV and UV radiation arealso obtained besides the desired EUV radiation. It is necessary,therefore, to keep this radiation away from the resists by means ofspectral filters.

So-called EUV beam tubes on synchrotron storage rings which emitmonochromatized EUV radiation constitute a highly stable EUV radiationsource for researching EUV lithography technology. Such EUV radiationsources emit very short radiation pulses (<1 ns) with repetitionfrequencies of a few MHz, so that these EUV sources are often referredto as quasi-cw sources. On EUV beam tubes on synchrotron storage rings,for the purpose of testing resists applied on slabs, individual fieldshave been irradiated sequentially with different radiation doses inorder to determine the influence of the radiation dose on the resist.Moreover, on synchrotron storage rings, a plurality of resist-coatedfields have also already been exposed simultaneously, a rapidly rotatingdiaphragm wheel arranged upstream of the resist layer in the beam pathperforming the function of a neutral wedge filter. The diaphragmapertures arranged radially on the wheel have different sizes, so thatthe individual fields are exposed to the radiation for different lengthsof time during each revolution. Reproducible radiation conditions on theindividual fields of the object are only possible with the diaphragmwheel because the EUV radiation source exhibits virtually steady-statebehavior on account of the high repetition frequency and radiates verystably.

Finally, irradiation experiments on resists have already been carriedout using low-power laboratory radiation sources for EUV radiation, ineach case only an individual field on the object having been irradiated.EUV laboratory radiation sources generate a dense and hot (>200 000° C.)plasma and emit the EUV radiation exclusively in very short pulses(typically 100 ns) with very low repetition rates (typically 10-1000Hz).

Taking this prior art as a departure point, the invention is based onthe object of providing a device for the test irradiation of objectscoated with photosensitive resists which, using an inexpensive radiationsource, enables an at least partly simultaneous irradiation of aplurality of irradiation fields on the object with varying dose in theshortest possible time and manages without complex and therefore costlyoptical systems in the beam path of the EUV radiation and in the case ofwhich a degradation of the optical elements in the beam path through EUVirradiation has no influence on the test result obtained.

In the case of a device of the type mentioned in the introduction, thisobject is achieved by virtue of the fact that

-   -   the EUV radiation source is a laboratory source for EUV        radiation,    -   the optical system has at least one filter for suppressing        undesirable spectral components of the radiation, in particular        of VIS, UV, DUV, VUV radiation, and also at least one mirror for        spectrally filtering the “in-band” EUV range,    -   the means for interrupting the beam path comprise a plurality of        closable diaphragm apertures which enable a temporal control of        the irradiation of irradiation fields that lie on the object and        are situated downstream of the diaphragm apertures, and    -   the at least one monitor detector is arranged downstream of the        optical system in the direction of the beam path and detects the        radiation dose during irradiation.

The laboratory source for EUV radiation is, by way of example, alow-power plasma-based source, e.g. an EUV lamp having a power of 100 Wand a pulse frequency of 50 Hz according to the HCT (hollow cathodetriggered) principle. The laboratory source reliably makes the requiredEUV radiation available over a long operating period.

The plasma of the laboratory source emits highly broadband radiationthat also contains DUV, VUV, UV and VIS radiation besides the desiredEUV radiation. In order to suppress these undesired spectral componentsof the radiation, the b optical system preferably has a spectral filter.The filter may comprise for example a thin metal film (e.g. a 150 nmthick zirconium film on a supporting grating). The filter is preferablysituated at the exit opening of the laboratory source. By means of thisarrangement, the filter prevents contaminants from the laboratory sourcefrom passing into the receiving chamber for the object to be irradiatedand from soiling parts situated there.

The optical system has the further task of ensuring that the irradiationis effected only with the “in-band” EUV radiation with a wavelength of13.5 nm. A multilayer mirror, in particular, is suitable for filtering.

The component parts of the optical system have the effect thatpractically only the desired EUV radiation impinges on the object.

The compact optical system of the device according to the invention, inparticular with only a filter and a mirror, enables a very smalldistance between the EUV laboratory source and the object to beirradiated with homogeneous irradiation of all the irradiation fields.The small distance means that it is possible to utilize a large solidangle of the thermal emission of the plasma even without a complexcondenser.

The diaphragm apertures that are closable according to the inventionpermit an at least partly simultaneous irradiation of the irradiationfields defined on the object through the diaphragm apertures. All theirradiation fields are initially irradiated in parallel until individualdiaphragm apertures are closed after reaching the target dose for theassigned irradiation field. A considerable gain in time is therebyachieved when testing the influence of the irradiation dose on aphotoresist.

The diaphragm apertures are preferably arranged in a planar plate andhave a diameter of 5 mm, by way of example. With 20 diaphragm aperturesof this type, the test duration for a photoresist can be reduced almostby a factor of 20 compared with individual irradiations with differentradiation doses.

After a calibration that is carried out beforehand, the monitordetectors arranged downstream of the optical system permit an exactmeasurement of the irradiation dose of the individual irradiationfields. By way of example, a plurality of photodiodes (Schottky type)may be used as monitor detectors. The signals supplied by the diodes arepreferably averaged in order to improve the measurement accuracy. Bycontinuously detecting the irradiation dose during irradiation, theirradiation of the irradiation fields can be carried out with preciselydefinable desired values for the irradiation dose.

The monitor detectors are preferably arranged between the optical systemand the closable apertures; they are expediently situated as near aspossible to the object to be irradiated. This arrangement of the monitordetectors makes the device insensitive to the degradation of the opticalsystem.

As already mentioned in the introduction, the entire beam path has to beguided up to the object under vacuum conditions. Therefore, the chamberfor receiving the object is designed for and evacuated to a negativepressure of 10⁻⁶ m bar for example. It is separated from the dischargechamber of the laboratory source by a window having an opening for thepassage of the radiation, a filter of the optical system, for example inthe form of a metallic film, being situated, in particular, in thewindow. This prevents contamination of the receiving chamber. Thereceiving chamber preferably has a dedicated pump system and, when theobject to be irradiated is being handled, is separated by means of aslide valve from the laboratory source and preferably also the regionfor receiving the optical system.

In order to obtain an irradiation that is as homogeneous as possible inthe individual irradiation fields, all the diaphragm apertures arearranged in one plane and the irradiation fields produced on the objectthrough each diaphragm aperture do not overlap. The irradiation fieldsare preferably arranged parallel to the plane of the diaphragmapertures.

The object coated with photoresist is, in particular, a silicon wafer,for example a 6 inch wafer having a thickness of 650 μm and having 20irradiation fields defined by the diaphragm apertures. A mount issituated in the receiving chamber and receives the wafer in such a waythat the EUV radiation impinges on the photoresist coating thereof.

In an expedient refinement of the invention, the laboratory source emitsradiation pulses having a duration of less than 1 is, in particular 100ns, with a repetition rate of between 1 and 10000 Hz, in particular1-5000 Hz. The radiation of the laboratory source originates from athermally emitting plasma, in particular from a laser-generated ordischarge-generated plasma or from an electron beam.

Preferably, a thin metal film, in particular a zirconium film having athickness of less than 200 nm but more than 100 nm, is arranged in thebeam path as filter for suppressing undesirable visible to VUVradiation. The film transmits up to 50% of the desired EUV radiation,while the undesirable radiation is suppressed by a factor of >1000.

Each mirror for spectrally filtering the “in-band” EUV range ispreferably configured as a multilayer mirror, in which case the mirrormay be embodied as a plane mirror or as a curved mirror. The multilayermirrors reflect up to 70% of the incident radiation in a narrow spectralband in the EUV range, while radiation that does not lie in this narrowband is almost completely absorbed by the multilayer mirror.

The diaphragm apertures are preferably closed by means of a flat slidewhich is arranged such that it can be displaced in a plane parallel tothe plane of the diaphragm apertures and has a contour enablingsuccessive opening or closing of the diaphragm apertures. The contour isstaircase-shaped, in particular, thereby enabling a row-by-row openingor closing of the diaphragm apertures arranged in rows. The flat slideas closure for all the diaphragm apertures, with only one mechanicalcomponent, constitutes a solution that is highly expedient in terms ofconstruction and control technology.

Further advantages and effects of the invention and also the operatingprocedure thereof emerge from the following description of an exemplaryembodiment with reference to the figures.

In the figures:

FIG. 1 shows the spectrum of the radiation generated by the EUVradiation source

FIG. 2 shows a basic illustration of the device according to theinvention for the test irradiation of objects coated with photosensitiveresists

FIG. 3 shows a diaphragm system with a flat slide arranged in the deviceaccording to FIG. 2

FIG. 4 shows an irradiation function with a variation of 50% withdifferent exponents, and

FIG. 5 shows an illustration of the film thickness of a resistapplication as a function of the dose of a test irradiation.

The device for EUV test irradiation serves for examining a photoresist(resist) for lithography in the range of EUV radiation, i.e. at awavelength of 13.5 nm, with 20 different radiation doses in one workoperation. In this case, the intention is to determine the removal ofthe photoresist after development and the sharpness of the imagedstructures depending on the dose.

The device for EUV test irradiation comprises an EUV laboratory lamp(1), which generates a radiation having a spectrum according to FIG. 1.Via a horizontally oriented beam tube (2) with an exit opening (3), thelikewise horizontally oriented beam path (4) leaves the EUV laboratorylamp (1).

A beam tube slide unit (5) is arranged at the exit opening (3). The beamtube slide has a passage into which a 150 nm thick zirconium film isinserted, which can be moved into the beam path (4) by means of theslide. The slide, which is movable transversely with respect to the axisof the beam path (4), permits the zirconium film to be completely movedout of the cross section of the beam tube (2), so that the exit opening(3) is completely closed by the beam tube slide, which incidentally iscomposed of metal. Furthermore, a turbomolecular pump (6) is arranged atthe beam tube (2) and generates a vacuum of approximately 10⁻³ mbar inthe EUV lamp (1) with a xenon atmosphere being maintained.

The beam tube slide unit (5) is adjoined by a hollow-cylindrical elbow(7), which receives a deflection mirror (8). The deflection mirror (8)is arranged in the interior of the elbow in the outer region of theangled-away portion in such a way that the horizontally impinging beampath (4) is deflected by 902 into a wafer chamber (9), which isdesignated in its entirety by (9). A mirror receptacle (11) carries andfixes the deflection mirror (8). It is pointed out that theconstructionally expedient angle of incidence of the EUV radiation of452 illustrated in the exemplary embodiment can readily be varied.

The elbow (7) is adjoined by the wafer chamber (9), which comprises ahollow-cylindrical beam tube (12) and also a receiving space (13) forthe resist-coated wafer. The beam path (4) propagates proceeding fromthe deflection mirror (8) through the beam tube (12) in the direction ofa diaphragm system (15). The wafer is oriented with its resist surfacein the direction of the diaphragm system (15), so that the EUV radiationthat passes through the diaphragm system falls onto the resist coatingof the wafer. The closure of the diaphragm apertures of the diaphragmsystem (15) is driven by a stepper motor (14).

A further turbomolecular pump (17) is arranged laterally at thereceiving space (13) and, during the exposure, ensures that a pressureof 10⁻⁶ mbar is maintained in the elbow (7) and also the wafer chamber(9).

Three photodiodes (18) are situated in the direction of propagation ofthe beam path (4) of the EUV radiation laterally in the diaphragm system(15), which photodiodes can be discerned in FIG. 3 and detect theradiation energy of the individual radiation pulses of the EUV lamp (1),the radiation energy being proportional to the charge generated in thephotodiodes (18). The photodiodes are arranged at the least possibledistance from the diaphragm apertures in the diaphragm system, but insuch a way that they are not concealed by the motor-driven closure.

Finally, the device for EUV test irradiation has a further slide (19)arranged between the elbow (7) and the beam tube (12) of the waferchamber (9). If the slide (19) is closed, the wafer chamber (9) iscompletely partitioned from the EUV lamp (1) and the interior space ofthe elbow (7).

FIG. 3 illustrates the construction of the diaphragm system, which isdesignated in its entirety by (15) and has a perforated mask (21) with 5rows each having 4 diaphragm apertures (22). The EUV radiation passingthrough each diaphragm aperture (22) defines a demarcated irradiationfield on the resist layer (16) of the wafer. The distance between waferand diaphragm system (15) and also the distance between the diaphragmapertures (22) are designed such that the irradiation fields do notoverlap. As a result, the diaphragm system (15) produces twentydemarcated irradiation fields having a diameter of approximately 5 mm onthe surface of the wafer coated with photoresist.

A flat slide (24) having a staircase-shaped contour (23) at the end issituated laterally beside the perforated mask (21). On the opposite sidefrom the contour (23), the flat slide (24) is connected to the steppermotor (14) illustrated in FIG. 2. By moving the flat slide (24) in thedirection of the arrow (25), it is possible for the diaphragm apertures(22) to be mechanically closed one after the other row by row. Theconsequence of this is that the irradiation fields defined by theindividual diaphragm apertures (22) acquire individual irradiationtimes.

During the irradiation of the coated wafer, the slide of the beam tubeslide unit (5) is pushed in such that the beam path passes through thezirconium filter. In this case, the filter has two functions:

-   1. Holding back radiations having wavelengths of greater than 20 nm.    At wavelengths of greater than 20 nm, the transmissivity of the    zirconium filter is less than 10%.-   2. Separating the xenon atmosphere in the EUV lamp (1) from the    region which is formed by the elbow (7) and the wafer chamber (9)    and into which no xenon gas should pass. The zirconium filter is    stable enough to withstand the pressure difference between the EUV    lamp (1) and the aforementioned region.

The deflection mirror (8) is a multilayer mirror having, for example, 40layers of a silicon substrate with a period thickness of approximately10 nm. This mirror reflects a wavelength of 13.5+/−0.2 nm at an angle of452 into the beam tube (12) of the wafer chamber (9).

After the conclusion of the irradiation of the photoresist on the wafer,the slide (19) between the elbow (7) and the wafer chamber is closed. Asa result, the vacuum is preserved in the EUV lamp (1) and the elbow (7)if the wafer chamber (9) is ventilated in order to open the latter forexample for the purpose of removing the irradiated wafer. The slide (19)enables not only shorter evacuation times of the wafer chamber

(9) during wafer handling, but furthermore an effective protection ofthe sensitive optical system that is formed by the zirconium film in thebeam tube slide unit (5) and the deflection mirror (8) in the elbow.

The photodiodes (18) arranged in the beam path (4) in the perforatedmask (21) measure the radiation energy of the EUV radiation pulses inthat they generate a charge proportional to the radiation energy in thephotodiodes. The charge generated by the individual pulses is added upelectronically and cyclically interrogated by a controller (notillustrated in the figure). If the interrogation reveals that a specificradiation dose (desired value) has been reached, a control command isinitiated for the stepper motor (14), which moves the flat slide (24) inthe direction of the arrow (25) in order to close the next diaphragmaperture (22) row by row. The desired values that have to be reacheddepending on a target dose prescribed by the user (definition: a dosewhich the user of the test system assumes to be optimal for the resistto be examined) before the next diaphragm aperture (22) is closed formthe discrete points of an irradiation function. The individual desiredvalues are calculated according to the following formula:if  s ≠ 10  then:${s\quad\left( {F,{Exp},{Tar},{Var}} \right)} = {{{Tar}\quad\left( {1 + {{VB}\frac{{RF}}{RF}\left( \frac{{RG}}{10} \right)^{Exp}}} \right)\quad{where}\quad{VB}} = {{\frac{Var}{100}{and}\quad{RF}} = {F - 10}}}$otherwise: s  (F, Exp, Tar, Var) = Tar

The following are applicable in this case:

-   s The function value s is the desired value that has to be reached    before the next diaphragm aperture is closed.-   F The parameter F represents the currently closed field and lies in    the range of values from 1 to 20.-   Exp The parameter Exp is the exponent set by the user and has the    values of 1 to 5.-   Tar The parameter Tar is the target dose set by the user.-   Var The parameter Var is the variation range set by the user in    percent in the range from 1 to 100.

For Tar:=1.0 and Var:=50, the characteristic curves shown in FIG. 4result depending on the exponent Exp:=1 to 5 for the desired values s.It becomes clear that the density of discrete points around the targetdose Tar increases as the exponent Exp increases.

The irradiation with EUV radiation brings about a removal of the resistfilm after the development on the wafer. The relationship between doseand removal after the development is illustrated in the curve accordingto FIG. 5 using the example of a concrete resist. The value for theresidual thickness of the resist film falls sharply starting from aspecific dose. The minimum dose required for the irradiation of thisresist (approximately 6 mJ/cm² in the exemplary embodiment) can be readfrom the x axis. In this way, it is possible to determine the EUVradiation sensitivity of a photoresist for wafers in one work operation.

LIST OF REFERENCE SYMBOLS

EUV lamp 1 Beam tube 2 Exit opening 3 Beam path 4 Beam tube slide unit 5Turbomolecular pump 6 Elbow 7 Deflection mirror 8 Wafer chamber 9 — 10Mirror receptacle 11 Beam tube 12 Receiving space 13 Stepper motor 14Diaphragm system 15 Wafer with resist layer 16 Turbomolecular pump 17Photodiodes 18 Slide 19 — 20 Perforated mask 21 Diaphragm apertures 22Staircase-shaped contour 23 Flat slide 24 Arrow 25

1-17. (canceled)
 18. A device comprising: a chamber for receiving anobject; an EUV radiation source for directing a beam through a beam pathtoward the object, the EUV radiation source comprising a laboratorysource for EUV radiation; an optical system for filtering radiation fromthe EUV radiation source, the optical system having at least one filterfor suppressing undesirable spectral components of the radiation andalso at least one mirror for spectrally filtering in-band EUV radiation;a plurality of closable diaphragm apertures located in the beam path,the plurality of closable diaphragm apertures enabling a temporalcontrol of irradiation of irradiation fields that lie on the object andare situated downstream of the diaphragm apertures; and at least onemonitor detector arranged downstream of the optical system in thedirection of the beam path, the monitor detector detecting anirradiation dose during irradiation.
 19. The device as claimed in claim18, wherein the object comprises a wafer coated with photoresist and thechamber for receiving the object has a mount for the wafer.
 20. Thedevice as claimed in claim 18, wherein the EUV radiation of thelaboratory source originates from a thermally emitting plasma.
 21. Thedevice as claimed in claim 18, further comprising a thin metal filmarranged in the beam path as a filter for suppressing undesirablevisible to VUV radiation.
 22. The device as claimed in claim 21, whereinthe thin metal film comprises a zirconium film having a thickness ofless than 200 nm.
 23. The device as claimed in claim 18, wherein the atleast one mirror for spectrally filtering the in-band EUV radiation isconfigured as a multilayer mirror.
 24. The device as claimed in claim18, wherein the at least one monitor detector is situated at a distancefrom the object to be irradiated, the distance being less than half of adistance between the EUV radiation source and the object to beirradiated.
 25. The device as claimed in claim 18, wherein eachdiaphragm aperture is assigned a separate closure mechanism.
 26. Thedevice as claimed in claim 18, wherein each of the diaphragm aperturesis arranged in one plane and the irradiation fields that lie on theobject through each diaphragm aperture do not overlap.
 27. The device asclaimed in claim 26, wherein the diaphragm apertures are closable bymeans of at least one flat slide.
 28. The device as claimed in claim 27,wherein the flat slide is arranged such that it can be displaced in aplane parallel to the plane of the diaphragm apertures and has a contourenabling successive opening or closing of the diaphragm apertures. 29.The device as claimed in claim 28, wherein the flat slide has astaircase-shaped contour enabling a row-by-row opening or closing of thediaphragm apertures arranged in rows.
 30. A method for irradiating anobject, the method comprising: directing an EUV radiation beam from alaboratory source; filtering the EUV radiation beam to suppressundesirable spectral components of the radiation; spectrally filteringthe EUV radiation beam in an in-band EUV range; detecting a radiationdose during irradiation by the EUV radiation beam; and interrupting abeam path of the EUV radiation beam by closing ones of a plurality ofclosable diaphragm apertures, wherein each diaphragm aperture is closedat an instant when the detecting ascertains that the radiation dosecorresponds to a desired value.
 31. The method as claimed in claim 30,wherein the plurality of closable diaphragm apertures enable temporalcontrol of the irradiation of irradiation fields that lie on an objectbeing irradiated and are situated downstream of the diaphragm apertures,and wherein each diaphragm aperture is closed at the instant when thedetecting ascertains that the irradiation dose in the irradiation fieldassigned to the diaphragm aperture corresponds to a desired value. 32.The method as claimed in claim 31, wherein the detecting is performed bya plurality of monitor detectors and wherein signals of each monitordetector are added up in a controller and compared with desired valuesstored there for each diaphragm aperture and, upon reaching the desiredvalue for an irradiation field, the controller drives a drive assignedto the closure of the respective diaphragm aperture.
 33. The method asclaimed in claim 32, wherein the desired values are generated inautomated fashion by the controller.
 34. The method as claimed in claim32, wherein the desired values are generated by the controller on thebasis of parameters input by an operator.
 35. The method as claimed inclaim 32, wherein the desired values are input into the controller by anoperator.
 36. The method as claimed in claim 35, wherein the number ofparameters to be input by the operator is less than or equal to thenumber of irradiation fields and at least one parameter forcharacterizing a typical dose for the photoresist to be tested, aparameter for determining the variation range in percent and a parameterfor determining the dose profile are input, the variation range definingthe range between the highest and lowest value relative to the typicaldose and the dose profile defining the change in the dose between twosuccessively closed irradiation fields.
 37. A device for the testirradiation of objects coated with photosensitive resists, the devicecomprising: an EUV radiation source; an optical system for filteringradiation from the EUV radiation source; a chamber for receiving anobject, the object being in a beam path of the radiation; means forinterrupting the beam path onto the object; and at least one monitordetector arranged downstream of the optical system in the direction ofthe beam path; wherein: the EUV radiation source is a laboratory sourcefor EUV radiation; the optical system has at least one filter forsuppressing undesirable spectral components of the radiation and also atleast one mirror for spectrally filtering radiation in an in-band EUVrange; the means for interrupting the beam path comprises a plurality ofclosable diaphragm apertures that enable a temporal control of theirradiation of irradiation fields that lie on the object and aresituated downstream of the diaphragm apertures; and the at least onemonitor detector detects the radiation dose during irradiation.